PLCs have been linked together in communication networks to coordinate the operation of an entire manufacturing or process plant, or in other instances, to control the operation of cells or segments of the operation. A cell controller typically coordinates and manages the operation of the cell, which consists of the PLCs and other programmable machine controls, and is designed to perform a complete manufacturing or process related task. The PLCs transfer messages between themselves and possibly to a host computer that provides supervisory control or monitoring of the process. These messages usually contain status information indicating the condition of the various interlocks, sensors, and other controls that monitor and control the process.
Many methods have been utilized to increase quality and improve productivity in the manufacturing environment. One such method is the use of a special form of a bar graph known as a Pareto Chart. The chart, developed by Dr. Joseph M. Juran, is based upon the Pareto principle which states that 80 percent of the problems come from 20 percent of the causes. The bars of the chart are arranged in descending order by height, with the height representing the frequency or relative importance of the cause. The Pareto Chart provides a graphic illustration of the categorical causes and is useful for establishing priorities, for showing which causes contribute most of the problems, and for showing the most significant opportunities for improvement which will yield the greatest return if rectified. Different measurement scales on the charts can help identify the more important causes. In most cases, frequency of the cause may be the most important. However, if one particular cause results in a long downtime of the process, downtime may provide a more useful parameter to display.
Present manufacturing processes require an operator to manually enter into a log, the cause and the length of the downtime. Further calculations are required to compute probability of the cause of the downtime if multiple causes are possible and to generate the chart itself. These calculations are difficult to do on-line and in real-time since they require operator intervention.